ROM devices include memory cells for storing data, and peripheral circuitry to control the read operation from the memory cells. A memory cell stores a bit of data. Specifically, each memory cell composed of an N-type transistor has a threshold voltage indicating each bit of the data. For example, a high threshold voltage corresponds to a "1" bit, and a low threshold voltage corresponds to a "0" bit.
There are various methods for manufacturing ROMs. For example, U.S. Pat. No. 5,538,914 (Chiu et al.) describes a method for manufacturing a ROM device having LDD (lightly doped drain) regions. The method of Chiu et al. uses two programming masks for an NLDD (N-type lightly doped drain) implant and a PLDD (P-type lightly doped drain) implant. Since the method of Chiu et al. requires two programming masks to be modified based on the data, the method results in higher manufacturing cost and complexity.
U.S. Pat. No. 5,407,852 (Ghio et al.) also describes a manufacturing method of a ROM device. However, the method of Ghio et al. also uses two photoresist masks for ROM programming. Namely, a photoresist mask M1 in FIG. 2 of Ghio et al., and a ROM protection photoresist mask M2 in FIG. 4 of Ghio et al. should be modified in accordance with the data to be programmed. Therefore, Ghio et al. does not improve the complexity and cost of manufacturing associated with the two mask programming.